DISSOCIATION OF LONG AND VERY LONG-CHAIN FATTY-ACIDS FROM PHOSPHOLIPID-BILAYERS

Dissociation of fatty acids (FA) from and transbilayer movement (flip- flop) in small unilamellar phosphatidylcholine vesicles (SUV) were mon itored by measuring the pH inside the vesicle with an entrapped water- soluble fluorophore, pyranin. With a pH gradient imposed upon SUV prel oaded with FA, the rate of flip-flop of saturated very long chain FA ( C20:0, C:22:0, and C24:0) was shown to be fast (t(1/2) < 1 s); previou sly, we showed by stopped flow measurements that flip-flop of long cha in (14-18 carbons) FA is very fast [t(1/2) < 10 ms; Kamp, F., et al. ( 1995) Biochemistry , 34, 11928-11937]. The rates of dissociation of FA from SUV were evaluated by incorporating FA into donor vesicles and m easuring transfer to acceptor vesicles. The transfer was followed by c hanges in internal pH of either donor or acceptor vesicles with stoppe d flow (C14:0, C16:0, C17:0, C18:0, C18:1, and C18:2) or on-line (C20: 0, C22:0, and C24:0) fluorescence. All FA showed a single-exponential transfer process that was slower than the lower limits established for the rate of flip-flop, with t(1/2) of dissociation ranging from 20 ms for C14:0 to 1900 s for C24:0, The pseudo-unimolecular rate constants (k(off)) for dissociation of C14:0 to C26:0 showed a 10-fold decrease for each addition of two CH2 groups to the acyl chain and a Delta(Del ta G) of -740 cal/CH2. The dissociation rate constants for oleic acid (18:1) and linoleic acid (18:2) were 5 and 10 times faster, respective ly, than that of C18:0. The rates of dissociation for typical dietary FA are sufficiently rapid that complex mechanisms (e.g. protein-mediat ed) may not be required for their desorption from biological membranes . The very slow dissociation rates for C24:0 and C26:0 may accentuate their pathological effects in diseases in which they accumulate in tis sues.